Ferrous metal and in particular, stainless steels when subjected to working processes such as drawing, stamping and bending, become hardened and contain microstructural stresses which render further working difficult or impossible.
Stainless steels are those which contain at least 11% chromium. The chromium markedly increases the corrosion resistance of the steel because of the formation of a very thin invisible passivating surface layer of chromium oxide which effectively protects the underlying metal from further reaction. Austenitic stainless steels are those which contain substantial quantities of nickel in addition to the chromium. For example, a common austenitic stainless steel is American Iron and Steel Institute (AISI) Type 302 which contains nominally 18% chromium and 8% nickel as its major alloying elements. In addition, the Austenitic Stainless Steels show transformation of the microstructure to martensite under heavy working stresses. Annealing is a process whereby the metal is heated to a high temperature which results in relief of trapped stresses and work hardening and formation of a solid solution of carbon in the austenite. Austenitic stainless steels are usually annealed at temperatures of 1700.degree. to 2100.degree. F. (927.degree. to 1149.degree. C.) to minimize formation of chromium carbides which sensitize the steel to corrosion.
Annealing must be carried out in an atmosphere which causes minimal chemical alteration of the metal by diffusion of atmosphere components into the surface of the metal. Excessive oxidation produces green, brown or black discoloration. In bright annealing (e.g. under an atmosphere of hydrogen and nitrogen) oxidation must be held to a level where no visible alteration of the surface occurs. Carburizing atmospheres may cause the precipitation of carbides of chromium and other metals which sensitize the steel to corrosion. Pure hydrogen is usually technically satisfactory as an annealing atmosphere, but it is more expensive than some other gaseous combinations.
Mixtures of hydrogen and nitrogen have been employed as annealing atmospheres for stainless steel. A commonly used combination, consisting of 75% hydrogen and 25% nitrogen, results from the cracking of ammonia. The generation of this atmosphere requires equipment for vaporization of liquid ammonia, and for cracking it over a suitable catalyst at a high temperature. Labor and energy are required for the operation and maintenance of the atmosphere generator. Furthermore, great care must be taken to ensure that cracking is complete with no residual ammonia which may cause nitriding of stainless steel. Nitriding is undesirable since it may promote intergranular corrosion, and cause severe embrittlement of the stainless steel. Most industrially generated dissociated ammonia atmospheres contain between 50 ppm and 500 ppm of undissociated ammonia. Because of this, an industrial atmosphere produced by dissociating ammonia cannot be directly equated to a 75% H.sub.2 -25% N.sub.2 atmosphere in regard to nitrogen absorption in finished (treated) parts.
More recently, inexpensive by-product nitrogen has been used as a base for stainless steel annealing atmospheres. A typical atmosphere consists of nitrogen containing from 10 to 50% hydrogen. However, such atmospheres may give rise to even more severe intergranular corrosion than is experienced with cracked ammonia. The hydrogen component of the atmosphere is capable of reducing the thin protective film of chromium oxide and exposing bare metal which then reacts readily at the high temperature of annealing with molecular nitrogen in the atmosphere. Since these synthetic atmospheres contain a higher concentration of nitrogen than does cracked ammonia, the degree of nitriding may be even more pronounced.
It has been known for some time that addition of small amounts of water, that is slight humidification of the atmosphere, limits the uptake of nitrogen by stainless steel to an acceptable level. Water addition may range, by weight, from less than 0.1% to 0.5%, depending on the type of steel and the application. It has also been known that addition of trace quantities of oxygen to the atmosphere also prevents excessive nitriding by synthetic nitrogen/hydrogen mixtures prepared by the dissociation of ammonia. The mechanism for the effectiveness of water and oxygen in preventing nitriding of stainless steel during annealing operations has been identified as resulting from the formation or preservation of a thin chromium oxide layer through oxidation of the metal surface by oxygen or water. A description of the state of the art is set forth in the articles by N. K. Koebel appearing in the July 1964 edition of Iron and Steel Engineer pp. 81 through 93 and the December 1977 edition of Heat Treating pp. 14 through 19.
However, as practical means for the limitation of nitriding by annealing atmospheres, both oxygen and water have been difficult to use. Both are highly reactive toward stainless steel at elevated temperatures, and unless the quantity of inhibitor is controlled with extreme care, excessive attack of the metal with the resultant formation of unsightly dark metal oxide coatings will take place.
Further, water, being a liquid presents handling problems not encountered with gases. Since only a very small quantity of water is required, provision must be made for the accurate continuous measurement of a tiny volume. This may require elaborate mechanical equipment, subject to continual maintenance and attention. If one elects to add the water by humidification of a sidestream of furnace atmosphere provision must be made for an appropriate humidifying device held at a closely controlled temperature. Successful operation of the stainless steel annealing process therefore is dependent upon the proper functioning of a number of complicated and delicate pieces of control equipment.